3GPP Long Term Evolution, LTE, is the fourth-generation mobile communication technologies standard developed within the 3rd Generation Partnership Project, 3GPP, to improve the Universal Mobile Telecommunication System, UMTS, standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lowered costs. The Evolved UTRAN, E-UTRAN, is the radio access network of an LTE system. In an E-UTRAN, a User Equipment, UE, is wirelessly connected to a Radio Base Station, RBS, commonly referred to as an evolved NodeB, or eNodeB. An RBS is a general term for a radio network node capable of transmitting radio signals to a UE and receiving signals transmitted by a UE.
In 5G, i.e. 5th generation mobile networks, there will be evolvement of the current LTE system to 5G. One task for 5G is to improve throughput and capacity compared to LTE. This is achieved by increasing the sample rate and bandwidth per carrier. 5G is also focusing on the use of higher carrier frequencies i.e. above 5-10 GHz.
By carrier aggregation, CA, the LTE standard supports efficient use of multiple carriers, allowing data to be simultaneously sent and received over several different carrier frequencies i.e. frequency bands. There is also support for cross-carrier scheduling avoiding the need that the UE listen to all carrier-scheduling channels all the time. The CA solution relies on tight time synchronization between the carriers.
To enable similar benefits as in carrier aggregation also for cases where different base stations and/or antenna sites are used with relaxed backhaul latency requirements, 3GPP initiated work labeled LTE dual connectivity. LTE dual connectivity is a solution currently being standardized by 3GPP to support UEs connecting to multiple carriers to send/receive data on multiple carriers corresponding to different base stations and/or antenna sites, at the same time.
The Dual Connectivity solution standardized in Release 12 can enable additional possible features, such as Control Plane Diversity (or RRC diversity).
The Radio Resource Control (RRC) protocol handles the control plane signaling of layer 3 between the UEs and the E-UTRAN. RRC includes e.g. functions for broadcast of system information and mobility procedures e.g. handover.
There can only be one RRC connection open to a UE at any one time. However, the messages of the connection may anyhow be transmitted via different base stations on lower layers. Therefore, introduction of RRC diversity has been discussed within the LTE release 12 time frame. RRC diversity is a technique to enable the communication of RRC messages to a user equipment, UE, via anchor link and booster link. The general idea for RRC diversity downlink signaling is that control messages are signaled from both an anchor eNodeB and a booster eNodeB.
Benefits from RRC Diversity have also been reported for inter-frequency scenarios in LTE, mainly when one of the frequency layers had coverage issues. This inter-frequency scenario can easily be extended for 5G Dual Connectivity, where it is expected that one of the links will be on LTE and another on the new 5G air interface, possibly operating in much high frequencies (up to 10 Ghz, or in extreme cases 30 Ghz or 60 GHz). In that case, this higher frequency link is sometimes expected to have spotty coverage due to challenging propagation conditions. Considering that for 5G, reliability requirements will be tougher, some sort of RRC Diversity is very likely to be considered.
However, the existing concepts for dual connectivity do not fully exploit the potential benefits of dual connectivity and in particular the potential benefits enabled by dual connectivity for control signaling.
Hence, there is a need for solution further exploiting the benefits of dual connectivity.